1. Field of the Invention
The present invention relates to a projection display system, and more particularly, this invention relates to a display system that improves temporal artifacts of a projection display by using spatial light modulators controlled by converting binary control signals to non-binary control signals to rearranging distributions of operational states of the mirrors.
2. Description of the Related Art
After the dominance of CRT technology in the display industry for over the past 100 years, Flat Panel Display (FPD) and Projection Display technologies are now gaining popularity because of a small form-factor of the display control system while enabled to project and display images of greater size onto a bigger display screen. Among several types of projection displays, projection displays using micro-display are gaining recognition by consumers because of high performance of picture quality as well as lower cost than FPDs. There are two types of micro-displays used for projection displays in the market. One is micro-LCD (Liquid Crystal Display) and the other is micromirror technology. Because a micromirror device uses un-polarized light, a micromirror device has an advantage on brightness over micro-LCD, which uses polarized light.
Even though there are significant advances made in recent years on the technologies of implementing electromechanical micromirror devices as spatial light modulator, there are still limitations and difficulties when employing them to provide high quality images display. Specifically, when the display image is digitally controlled, the image quality is adversely affected due to the fact that the image is not displayed with a sufficient number of gray scales.
An electromechanical mirror device is drawing a considerable interest as a spatial light modulator (SLM). The electromechanical mirror device consists of “a mirror array” arraying a large number of mirror elements. In general, the mirror elements ranging from 60,000 to several millions are arranged on a surface of a substrate in an electromechanical mirror device. Referring to FIG. 1A, an image display system 1 including a screen 2 is disclosed in a reference U.S. Pat. No. 5,214,420. A light source 10 is used for generating light energy for illuminating the screen 2. The generated light 9 is further concentrated and directed toward a lens 12 by a mirror 11. Lenses 12, 13 and 14 form a beam columnator operative to columnate light 9 into a column of light 8. A spatial light modulator (SLM) 15 is controlled on the basis of data input by a computer 19 via a bus 18 and selectively redirects the portions of light from a path 7 toward an enlarger lens 5 and onto the screen 2. The SLM 15 has a mirror array arraying switchable reflective elements 17, 27, 37, and 47 consisting of a mirror 33 connected by a hinge 30 on a surface 16 of a substrate in the electromechanical mirror device as shown in FIG. 1B. When the element 17 is in one position, a portion of the light from the path 7 is redirected along a path 6 to lens 5 where it is enlarged or spread along the path 4 to impinge on the screen 2 so as to form an illuminated pixel 3. When the element 17 is in another position, the light is not redirected toward the screen 2 and hence the pixel 3 is dark.
Each of mirror elements constituting a mirror device for functioning as a spatial light modulator (SLM) and each mirror element comprises a mirror and electrodes. A voltage applied to the electrode(s) generates a coulomb force between the mirror and electrode, thereby making it possible to control and incline the mirror. And the mirror is “deflected” according to a common term used in this specification for describing the operational condition of a mirror element.
When a voltage applied to the electrodes for controlling the mirror deflects a mirror, the deflected mirror also changes the direction of a reflected light as a result of reflecting an incident light. The direction of the reflected light is changed in accordance with the deflection angle of the mirror. The present specification refers to a state of the mirror when a light of which almost the entirety of an incident light is reflected to a projection path designated for an image display as an “ON light”, while it refers to a light reflected to a direction other than the designated projection path for the image display as an “OFF light”.
And a state of the mirror that reflects the light of an incident light in a manner that the ratio of the light reflected to a projection path (i.e., the ON light) to that reflected in a shift from the projection path (i.e., the OFF light) is a specific ratio, that is, the light reflected to the projection path with a smaller quantity of light than the state of the ON light is referred to as an “intermediate light”.
According to a term of present specification, an angle of rotation in a clockwise (CW) direction is defined as a positive (+) angle and that of rotation counterclockwise (CCW) direction as negative (−) angle. A deflection angle is defined as zero degree (“0°”) when the mirror is in the initial state as a reference of mirror deflection angle.
Most of the conventional image display devices such as the device disclosed in U.S. Pat. No. 5,214,420 implements a dual-state mirror control that controls the mirrors at a state of either ON or OFF. The quality of an image display is limited due to the limited number of gray scales. Specifically, in a conventional control circuit that applies a PWM (Pulse Width Modulation), the quality of the image is limited by the LSB (least significant bit) or the least pulse width as control related to the ON or OFF state. Since the mirror is controlled to operate in either of the ON or OFF state, the conventional image display apparatus has no way to provide a pulse width for controlling the mirror that is shorter than the control duration allowable in accordance with the LSB. The least quantity of light, which determines on the basis of the gray scale, is the light reflected during the time duration based on the least pulse width. The limited number of gray scales leads to a degradation of the quality of an image.
Specifically, FIG. 1C shows an exemplary control circuit for controlling a mirror element according to the disclosure made in the U.S. Pat. No. 5,285,407. The control circuit includes a memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5 and M7 are p-channel transistors; while transistors M6, M8, and M9 are n-channel transistors. The capacitances C1 and C2 represent the capacitive loads in the memory cell 32. The memory cell 32 includes an access switch transistor M9 and a latch 32a, which is based on a Static Random Access Switch Memory (SRAM) design. The transistor M9 connected to a Row-line receives a DATA signal via a Bit-line. The memory cell 32-written data is accessed when the transistor M9 that has received the ROW signal on a Word-line is turned on. The latch 32a consists of two cross-coupled inverters, i.e., M5/M6 and M7/M8, which permit two stable states, that is, a state 1 is Node A high and Node B low, and a state 2 is Node A low and Node B high.
The mirror is driven by a voltage applied to the landing electrode abutting a landing electrode and is held at a predetermined deflection angle on the landing electrode. An elastic “landing chip” is formed at a portion on the landing electrode, which makes the landing electrode contact with mirror, and assists the operation for deflecting the mirror toward the opposite direction when a deflection of the mirror is switched. The landing chip is designed as having the same potential with the landing electrode, so that a shorting is prevented when the landing electrode is in contact with the mirror.
Each mirror formed on a device substrate has a square or rectangular shape and each side has a length of 4 to 15 um. In this configuration, a reflected light that is not controlled for purposefully applied for image display is however inadvertently generated by reflections through the gap between adjacent mirrors. The contrast of image display generated by adjacent mirrors is degraded due to the reflections generated not by the mirrors but by the gaps between the mirrors. As a result, a quality of the image display is degraded. In order to overcome such problems, the mirrors are arranged on a semiconductor wafer substrate with a layout to minimize the gaps between the mirrors. One mirror device is generally designed to include an appropriate number of mirror elements wherein each mirror element is manufactured as a deflectable micromirror on the substrate for displaying a pixel of an image. The appropriate number of elements for displaying image is in compliance with the display resolution standard according to a VESA Standard defined by Video Electronics Standards Association or the number in compliance with the television broadcast standards. In the case in which the mirror device has a plurality of mirror elements corresponding to WXGA (resolution: 1280 by 768) defined by VESA, the pitch between the mirrors of the mirror device is 10 um and the diagonal length of the mirror array is about 0.6 inches.
The control circuit as illustrate in FIG. 1C controls the micromirrors to switch between two states and the control circuit drives the mirror to oscillate to either the ON- or OFF-deflected angle (or position) as shown in FIG. 1A.
The minimum quantity of light controllable to reflect from each mirror element for image display, i.e., the resolution of gray scale of an image for a digitally controlled image display apparatus, is determined by the least length of time that the mirror can be controlled to hold at the ON position. The length of time that each mirror is controlled to hold at the ON position is in turn controlled by multiple bit words. FIG. 1D shows the “binary time periods” in the case of controlling an SLM by four-bit words. As shown in FIG. 1D, the time periods have relative values of 1, 2, 4, and 8 that in turn determine the relative quantity of light of each of the four bits, where the “1” is least significant bit (LSB) and the “8” is the most significant bit. According to the PWM control mechanism, the minimum quantity of light that determines the resolution of the gray scale is a brightness controlled by using the “least significant bit” for holding the mirror at the ON position during a shortest controllable length of time.
FIG. 2A shows an example of an insufficient number of grayscales, where the minimum step of brightness change is very large and the artifacts are well visible. FIG. 2 shows an example of an improved grayscale in which the artifacts are less visible.
As illustrated in FIG. 2A, when adjacent image pixels are displayed with great degree of different gray scales due to a very coarse scale of controllable gray scale, artifacts are shown between these adjacent image pixels. That leads to image degradations. The image degradations are specially pronounced in bright areas of display when there are “bigger gaps” of gray scales between adjacent image pixels. It was observed in an image of the girl shown in FIG. 3A that there were apparent artifacts on the forehead, the sides of the nose and the upper arm. The artifacts are generated due to a technical limitation that the digitally controlled display does not provide a sufficient number of gray scales. At the bright spots of display, e.g., the forehead, the sides of the nose and the upper arm, the adjacent pixels are displayed with visible gaps of light intensities. When the number of gray scales are increased, the image degradation will be much less even with only twice the number of gray scales as illustrated in FIG. 2B.
Since the micromirrors are controlled to have the fully on and fully off positions, the light intensity is determined by the length of time the micromirror is at the fully on position. In order to increase the number of gray scales of display, the speed of the micromirror must be increased such that the digitally controlled signals can be increased to a higher number of bits.
However, when the speed of the micromirrors is increased, a strong hinge is necessary for the micromirror to sustain a required number of operational cycles for a designated lifetime of operation. In order to drive the micromirrors supported on a further strengthened hinge, a higher voltage is required. The higher voltage may exceed twenty volts and may even be as high as thirty volts. The micromirrors manufactured by applying the CMOS technologies may not probably be suitable for operation at such higher range of voltages and therefore the DMOS or High Voltage MOSFET technologies may be required. In order to achieve higher degree of gray scale control, a more complicate manufacturing process and larger device areas are necessary when DMOS micromirror is implemented.
Conventional modes of micromirror control are therefore facing a technical challenge that the gray scale accuracy has to be sacrificed for the benefits of smaller and more cost effective micromirror display due to the operational voltage limitations.
There are many patents related to a light intensity control. These patents include U.S. Pat. Nos. 5,589,852, 6,232,963, 6,592,227, 6,648,476, and 6,819,064. There are more patents and patent applications related to different shapes of light sources.
These patents includes U.S. Pat. Nos. 5,442,414, 6,036,318 and Application 20030147052. The U.S. Pat. No. 6,746,123 discloses special polarized light sources for preventing light loss. However, these patents or patent application does not provide an effective solution to overcome the limitations caused by an insufficient number of gray scales in the digitally controlled image display systems.
Furthermore, there are many patents related to spatial light modulation that includes U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, and 5,489,952.
However, these inventions have not addressed or provided direct resolutions for a person of ordinary skills in the art to overcome the above-discussed limitations and difficulties. Therefore, a need still exists in the art of image display systems applying digital control of a micromirror array as a spatial light modulator to provide new and improved systems such that the above-discussed difficulties can be resolved. The largest difficulty to increase gray scale is that the conventional systems have only the ON and OFF states and the minimum ON time exists. The minimum ON time determines the height of the steps of gray scales shown in FIGS. 2A and 2B.
There is no way of providing the brightness lower than the step. If a level of brightness lower than the height of steps can be generated, then the number of gray scales increases and the degradation of picture quality reduced substantially. The previously mentioned pending application Ser. Nos. 11/121,543, 11/136,041 and 11/183,216 provide solutions to improve the number of grayscales.
Although the increase of grayscales can eliminate the artifacts of still pictures, it does not solve so called “temporal artifacts” of pictures in motion.
When a display is controlled digitally and pictures are in motion, temporal artifacts can be observed as illustrated in FIG. 3B showing the same picture but in motion. Some lines in the upper arm and the forehead are even more visible than the artifacts in FIG. 3A.
An example of 8-bit digital signal is illustrated in FIGS. 4A, 4B and 4C. The least significant bit (LSB) of the incoming signal is shown as the left smallest block and the most significant bit (MSB) is shown as the largest block in the right side of FIG. 4A, in which each bit has binary time width that is called a Binary Pulse Width Modulation (PWM). The LSB has the minimum unit of time width and the MSB has the maximum time width in a frame. If all of the bits are OFF, the output will be zero. All the bits from the first to seventh are ON and the eighth bit is OFF, and the output is 127 as in FIG. 4B. All the bits from the first to seventh are OFF and the eighth bit is ON, and the output is 128 as in FIG. 4C.
These two signals differ numerically by one (“1”); their displayed times, however, are very different. The first one is in the first half of the frame; and the second one is in the latter half of the frame.
Human eyes have a certain response time to an incoming light. If two light pulses enter eyes in a short time interval, the eyes recognize them as a single light pulse. As the time interval becomes larger, the eyes start recognizing them as two light pulses. This response time of human eyes are considered as about 20 milliseconds or 50 Hz. Because we often recognize the flicker of TVs in Europe where TVs are scanned in 50 Hz, but we do not recognize the flicker in the US where TVs have a scanning frequency of 60 Hz.
Because of the response time of human eyes and the frame frequency being higher than 60 Hz, the time interval between the pulse in FIG. 4B and that in FIG. 4C is short enough, and the integration of light by human eyes makes the picture look smooth enough. However, if the two pixels having 127 and 128 brightness levels as shown in FIGS. 4B and 4C are adjacent to each other and the pattern is moving, another type of artifacts take place. This is called “temporal artifacts”.
FIG. 5 illustrates the reason why these artifacts take place. When a picture is in motion, the viewer's eyes chase the object. The integration of incoming light by the viewer's eyes is performed with two adjacent pixels rather than a single pixel as the case of still pictures. If the display is controlled with digital signals and a pixel is ON in the latter half of a frame period as shown in FIG. 4B and the adjacent pixel is ON in the first half of the frame, the light integration in the viewer's eyes will have a duplicated intensity as the brighter lines shown in FIG. 5. This phenomenon has been well known as temporal artifacts of a plasma display. However, these artifacts are not limited to a plasma display and can take place with any digitally controlled displays. The present invention provides the solutions to eliminate or reduce temporal artifacts.
According to above discussions, FIGS. 1A through 1D illustrate conventional display and FIGS. 2A and 2B illustrate the definition of gray scale and the artifacts arising from low gray scale representation when applying the conventional display systems of FIGS. 1A to 1D. FIG. 3A shows an example of the artifacts using a photo of a female wherein there are visible unnaturally bright lines or areas in the forehead and the upper arm due to an insufficient number of grayscales. FIG. 3B shows an example of temporal artifacts that show the unnatural lines that are clearly observable in a display image. FIG. 5 illustrates the reason why these unnatural lines are created. As shown in FIGS. 4B and 4C, a single increment of brightness can cause a major separation of the ON times between two adjacent pixels, because the brightness level of 127 having its ON time in the first half of a frame and the brightness level of 128 will have the ON time in the latter half of the frame. When the pattern, such as the female object, is moving in the screen, the viewer's eyes chase the pattern. Because of this movement of view points, the integration of incoming light will no longer take place at the same pixels, but the integration will take place over adjacent pixels. In the case of FIG. 5, the brightness of the 4th pixel in the first half of the frame and the brightness of the 5th pixel in the latter half of the frame are added. This causes the brightness at the 5th pixel unnaturally brighter than the adjacent area. The artifacts shown in FIG. 3B are extremely uncomfortable to a viewer. The elimination of this type of artifacts is highly desirable.
Accordingly, improvements in the display systems are necessary to prevent such a degradation of image quality. Therefore, a need still exists to further improve the image display systems such that the above discussed difficulties and limitations can be resolved.